Dev gw

[YannIntVeld]

Dear all,

As discussed with Nils and Alex, these are the changes I made to the GW part of the tprf code. The main changes I made are:

• Implementing the GW self-energy for static interactions
• Collecting the various Matsubara and real-freq. self-energy functions into one overloaded gw_sigma function
• Overloading the dynamical_screened_interaction functions and implementing the real-freq. equivalents
• Adding documentations to functions that still needed them (mainly the real-freq. implementations)

While working on this I also discovered that the lattice_dysong functions in the gf.cpp file were not thread safe. I fixed this issue here by initializing the Green functions to 0.

[Wentzell]

Thank you @YannIntVeld for all these improvements!

I iterated on the PR and pushed a few simplifications. From my side we can go ahead with the merge!